Ammonium salts of alkyl triarylborates as coinitiators and the synthesis thereof are known. In the synthesis, which can be performed efficiently on industrial scale by the following route:
a mixture of alkyltriarylborates of the formula (A)
and tetraarylborates of the formula (B)
is always obtained, since the hydroboration requires tribromoborane as catalyst. If, in contrast, the mixture of alkyl triarylborates and tetraarylborates obtained in the synthesis is used as coinitiators in holographic media, inadequate long-term stability and photochemical bleachability are found. Materials of this kind are therefore unsuitable for industrial utilization. However, the separation of this mixture is complex and inevitably leads to considerable substance losses.
It was therefore an object of the present invention to provide a photopolymer composition which comprises a coinitiator which is simple and inexpensive to prepare and, at the same time, is suitable for producing holographic media having good long-term stability and photochemical bleachability. An efficient process for preparing suitable coinitiators was likewise to be provided.